In recent years, many users may use portable information devices, for example a notebook personal computer (PC), a cell phone or a personal digital assistant (PDA), in various environments due to the rapid development of information industries. However, numbers and letters on keyboards or keys of the portable information devices may not be clear in a darker environment, and thus the portable information devices are difficult to be operated, or even may damage the vision due to discerning the numbers and letters hardly. Therefore, the development of luminous keyboards may reduce the inconvenience of using the keyboards in a darker environment. Further, the information devices with a luminous keyboard may be more artistic by disposing various illumination elements and thus may be more popular.
FIG. 1 illustrates a structural cross-sectional view of a conventional luminous keyboard. Referring to FIG. 1, the luminous keyboard 1 comprises a reflector 11, a light guide plate 12, a metal base plate 13, a membrane circuit board 14, a plurality of keys and an illumination element 16. Herein the membrane circuit board 14 comprises a lower board 141, an upper board 142 and a middle board 143 disposed between the lower board 141 and the upper board 142. In addition, each of the lower board 141, the middle board 143 and the upper board 142 may be made of a transparent light guide material, such as polycarbonate (PC) or polyethylene terephthalate (PET).
FIG. 2 illustrates an exploded perspective view of a membrane circuit board of the luminous keyboard as shown in FIG. 1. Further referring to FIGS. 1 and 2 together, the lower board 141 has a first circuit layout 1411 composed of a plurality of silver glue circuits 14111 and a plurality of lower pads 14112. The upper board 142 has a second circuit layout 1421 composed of a plurality of silver glue circuits 14211 and a plurality of upper pads 14212. Furthermore, the middle board 143 has a plurality of pad openings 1431 corresponding to the plurality of lower pads 14122 and the plurality of upper pads 14212. Herein, each of the upper pads 14212 and the corresponding one of the lower pads 14112 form a membrane switch 144 together. Besides, each of the keys 15 has a cap 151 and an elastomer 152, and the elastomer 152 is disposed between the cap 151 and the membrane circuit board 141.
When any one of the keys 15 is pressed to move downwards, the cap 151 presses the elastomer 152 to let the elastomer 152 push the corresponding one of the upper pads 14212. Thus, the corresponding one of the upper pads 14212 contacts the corresponding one of the lower pads 14122 via the corresponding one of the pad openings 1431 to switch on the corresponding one of the membrane switches 144, and thus the luminous keyboard 1 generates a corresponding key signal accordingly. Furthermore, the position of the cap 151 returns upwards due to an elastic force of the elastomer 152 when a user releases the cap 151.
Besides, the illumination element 16 is disposed at a side of the light guide plate 12 for providing light beams emitting into the light guide plate 12. The light guide plate 12 is disposed on the reflector 11 and a lower surface thereof has a plurality of light guide points 121 used to aggregate and scatter the light beams. The plurality of light guide points 121 are disposed corresponding to the positions of the plurality of keys 15. The metal base plate 13 has a plurality of apertures 131 corresponding to the plurality of light guide points 121. Herein, the light beams are able to be diffused all over the light guide plate 12 after the light beams are emitted into the light guide plate 12, and the plurality of light guide points 121 scatter the light beams upwards and downwards due to made of ink material(s). The light beams scattered upwards pass through the plurality of apertures 131 and then are projected to the membrane circuit board 14 and the plurality of keys 15, and the light beams scattered downwards are further reflected upwards by assistance of the reflector 11, and thus the light beams provided by the illumination element 16 may be fully utilized. Accordingly, the plurality of keys 15 may be illuminated. However, the conventional luminous keyboard 1 has following disadvantages:                1. The light beams scattered downwards by the plurality of light guide points 121 may be reflected upwards by assistance of the reflector 11. However, the material of the reflector 11 of the conventional luminous keyboard 1 may allow the light beams to pass through. Therefore, some of the light beams scattered downwards by the plurality of light guide points 121 may pass through the reflector 11 and thus being wasted.        2. The first circuit layout 1411 is disposed at an upper surface of the lower board 141 and the first circuit layout 1411 is composed of the plurality of silver glue circuits 14211, wherein the silver glue circuits 14211 has a property of aggregating and scattering the light beams. Therefore, the silver glue circuits 14211 of the first circuit layout 1411 may be converted into a luminous status after the light beams provided by the illumination element 16 emitting into the lower board 141 via the plurality of apertures 131 of the metal base plate 13. However, most of the silver glue circuits 14211 are not correspondingly disposed with the plurality of keys 15. Therefore, the light beams initially provided for illuminating the plurality of keys 15 may be wasted due to the property of the silver glue circuits 14211, and thus the brightness of the plurality of keys 15 may not be enough.        
Accordingly, it is necessary to improve the light utilization efficiency for the conventional luminous keyboard 1.